1. Field of the Invention
This invention relates to an improvement in an anti-skid control apparatus.
This invention also relates to a highly improved anti-skid system to vary the wheel cylinders pressure increasing speed in accordance with the road condition during brake pressure re-application. More specifically, the gradual increase of the brake pressure is desirable so as to avoid the wheel locking on a low-.mu. road, while on a high-.mu. road it is desirable to rapidly increase the brake pressure until reaching a predetermined level and to thereafter gradually increase the brake pressure so as to get sufficient brake force.
In accordance with the invention, it is possible to get the friction coefficient .mu. of the road based upon the vehicle deceleration when the anti-skid control is to be applied. That is, when the deceleration is great upon the anti-skid operation the vehicle is presumed to be on a high-.mu. road. When the deceleration is small the vehicle is presumed on a low-.mu. road. Therefore, the selection of the restriction of operational fluid for the actuator in accordance with the magnitude of vehicle deceleration will result in the rapid increase of brake pressure on a high-.mu. road and a gradual increase thereof on a low-.mu. road.
In the anti-skid control apparatus of the invention, a valve actuated by sensing a predetermined fluid pressure level during the pressure increasing stage and a valve actuated by sensing a predetermined magnitude vehicle deceleration are arranged in a row within a fluid passage.
2. Description of the Prior Art
In order to vary the pressure increasing speed by the restriction of the operational fluid of an actuator in accordance with the magnitude of vehicle deceleration, it is a conventional practice to provide an electromagnetic valve for the restriction of fluid in the fluid passage. According to such a conventional system, it is necessary to provide a sensor for detecting the vehicle deceleration, electrical lines for setting the change-over point of the electromagnetic valve in accordance with the output of the sensor, and other interrelated structure. Thus, such a conventional system has been found to be costly since the sensor and the electric lines as well as the electromagnetic valve are expensive.
Further, in order to avoid early wheel locking and get a gradual increase of pressure on a low-.mu. road, it is necessary to sufficiently restrict the fluid passage. However, when such restriction is effected from the initial stage of brake re-application, the starting time of the brake re-application will be considerably retarded. The time period for pressure decrease is likely to be of considerable length because the retard return of wheel rotation though the piston stroke for pressure decrease may be small in view of the low brake pressure at the starting of pressure decrease, particularly on a low-.mu. road. In such a case, the fluid pressure in the actuator is excessively varied and the power piston is also excessively moved. Thus upon brake re-application the excessive movement is, at first to be compensated, so that the compensating time is elongated when the fluid restriction is small and the response to the computer signal for re-pressurizing is reduced. As a result, it is impossible to effectively control the anti-skidding and the stopping distance of the vehicle is unavoidably enlarged due to the retard of brake force application.
In order to overcome these drawbacks it is conventional to employ a solenoid valve for selection of the fluid restriction which may switch-over the rapid pressure increase to the gradual pressure increase and to provide electric circuits for setting the switch-over time based upon the vehicle deceleration by an electric acceleration responsive sensor. However, such conventional apparatus is expensive since the solenoid valve, the acceleration responsive sensor and the electric circuits are expensive per se.